A rolling mill stand can comprise hydraulic devices for adjusting the rolling gap and the pressure applied by, for example, a back-up roll or support roll to the working roll of such a stand.
In a common construction of such a rolling-mill stand, the adjustment of the gap and rolling pressure is effected by a threaded spindle which acts via the hydraulic piston-and-cylinder arrangement upon the journal block of the pressure or backing roll which, in turn, bears upon the working roll. The hydraulic cylinder is pressurizable with a hydraulic medium and sealing rings or, more generally, a sliding packing or sealing device is provided between the cylinder and piston walls.
Such adjusting cylinders are used for the rapid adjustment of the rolling gap and it is desirable to so construct and arrange the hydraulic adjusting device that it can operate with high adjusting speeds and, especially, can sustain high adjusting accelerations.
The piston-cylinder unit of such a hydraulic adjusting system is generally subjected to extremely high fluid pressures.
Such high fluid pressures have a tendency to expand the cylinders and thereby change the dimensions of the gap between the piston and cylinder.
The sliding packing or seal between the piston and cylinder has generally only a limited capacity to compensate for this change in the gap dimensions. Furthermore, there is always the danger that the change in the gap size between the piston and the cylinder will result in leakage and subject the sliding seal or packing, as a result of these changes in gap dimensions, not only to variable axial stresses, but also to variable radial stresses, thereby reducing the operating life of the packing. A reduction in the useful life of the packing means that the rolling stand can only be operated for limited periods before maintenance operations must be performed and increases the down time of the stand while reducing the duty cycle thereof.
Another result of the tendency for the high pressures to expand the cylinders radially is that the friction effects between the sliding packing and the cylinder wall will change. This is undesirable and creates difficulties in precision adjustment of the rolling gap.